It now seems certain that many of the adaptations of vertebrates to environmenal variables involve the pineal apparatus. Light, especially the duration of the photoperiod, appears to be a paramount environmental cue which regulates the physiological state of the animal and pineal components form part of the system that mediate its effects. Rhythmic behavior, diurnal color changes, thermal adaptations, osmotic adjustments and metabolic shifts are some of the kinds of adaptative responses that may involve the pineal complex to a significant degree. Such functions are best investigated, in most instances, in lower vertebrate groups where the physiological mechanism are often more amenable to manipulations. From our studies, collectively, we expect to understand pineal functions better by developing a trans-vertebrate perspective. Through the lower forms, one should obtain a better conception sooner of the role of the pineal organ of the higher forms, including the little-understood human pineal gland.